<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
       .child{
          width: 500px;
          height: 100px;
          background: #000;
          margin-top:10px;
          margin-bottom: 10px;
       }
       .bfc{
         overflow: hidden;
       }
    </style>
</head>
<body>
    <div class="child"></div>
    <div class="bfc">
       <div class="child"></div>
    </div>
</body>
</html>